I'm having this same issue.
All my Opera that were updated to 50 stopped syncing the bookmarks.
Any new installation is unable to sync.
This is very very annoying.
On Windows I don't get any message error. But the icon on top right has an small orange exclamation point.
And on the menu it continuously displays "Signing in ...".
It is not an password/username issue. I'm able to login to the website.